1. Field of the Invention
The present invention relates generally to a chip type varistor functioning as a voltage non-linear resistor, and more particularly, to a chip type varistor so constructed as to obtain varistor characteristics in the interface of an inner electrode and a semiconductor ceramics layer.
2. Description of the Prior Art
Recently in the field of electronic equipments such as a communication device, miniaturization and integration of electronic components have proceeded. Correspondingly, also in a varistor, the demands for miniaturization and a lower voltage have been increased. A monolithic type varistor as shown in FIG. 5 has been conventionally proposed as one meeting the demands (Japanese Patent Laid-Open Gazette No. 302496/1989).
In this monolithic type varistor 20, a pair of inner electrodes 22 and 23 are embedded in a sintered body 21 formed by laminating a lot of semiconductor ceramics layers. In addition, only respective one edges 22a and 23a of the inner electrodes 22 and 23 are led out to side surfaces 21a and 21b of the above described sintered body 21, and are connected to outer electrodes 24 and 25 formed on the side surfaces 21a and 21b. Non-connected type inner electrodes 27 which are not connected to the above described outer electrodes 24 and 25 are embedded in semiconductor ceramics layers 28 between the above described inner electrodes 22 and 23, and each of the non-connected type inner electrodes 27 is sealed in the sintered body 21.
In this monolithic type varistor 20, varistor characteristics are obtained in each of the interfaces of the above described inner electrodes 22 and 23 and non-connected type inner electrodes 27 and the semiconductor ceramics layers 28. In addition, the number of grain boundaries in the direction of thickness of the ceramics layers 28 among the above described electrodes 22, 23 and 27 is set to not more than two, and a varistor voltage is controlled by selecting the number of ceramics layers 28 laminated.
Meanwhile, in the above described conventional monolithic type varistor, the inner electrodes or the non-connected type inner electrodes are formed on a plurality of ceramic green sheets and then, the plurality of ceramic green sheets are laminated and cofired, thereby to obtain the sintered body 21. Consequently, mesh-shaped holes are easily generated in the inner electrodes and the non-connected type inner electrodes after sintering by the shrinkage of a metal constituting the above described inner electrodes or the like and the evaporation of organic matter at the time of sintering. As a result, a semiconductor crystal grows in the direction of thickness in the sintered body through the holes, thereby to encounter the problems of causing the variation of a varistor voltage and increasing a leakage current. In addition, there also arises the problem of decreasing the surge current withstand because current concentrations are easily caused in the above described semiconductor crystal portion which has grown.